1. Field of the Invention
This invention is directed to high resolution fabrication techniques, in general, and, more particularly, to a mask and the method of making same for use in implementing these techniques.
2. Prior Art
In fabricating many devices of high resolution, such as magnetic bubble domain structures, semiconductor device structures and the like, various techniques have been developed to improve the density and the resolution of these structures. These techniques include E-beam and x-ray lithography. These techniques have been used in replicating or fabricating microcircuits capable of resolution of better than 0.1 micron. In the past, however, the resolution has been limited primarily by two factors, viz. (1) the required thickness of a heavy metal absorber for absorbing the radiation in order to provide adequate contrast and (2) the range of the electrons or photons produced by absorption of the x-rays or electrons in the mask resist. That is, the thickness of the absorber which is required generally limits the achievable linewidth inasmuch as the thickness/width aspect ratio of the absorber is generally limited to one or two. Moreover, photoelectrons generated by the absorption of the x-ray or E-beam creates problems in resolution and the like.
A commonly used source for soft x-ray lithography is aluminum. A 6 dB gold absorber thickness is approximately 3,000 angstroms for AlK radiation while the effective photoelectron range in typical resist is greater than 400 angstroms. Also it is known that the commonly used Mylar membrane having a thickness of approximately 6 microns transmits only about 50% of the AlK radiation. These characteristics provide poor resolution for fine-line lithographic purposes. To improve resolution, a CuL source has been proposed. This source requires only 850 angstroms of gold to produce a 6 dB absorber as well as having an effective photoelectron range of about 200 angstroms. While this would improve resolution by a factor of 2, the exposure time of, for example, x-rays, would be substantially increased inasmuch as the 6 micron Mylar mask transmits only about 10% of the CuL radiation. In addition, the Mylar mask does not attenuate the continuum radiation as effectively as the CuL. Consequently, contrast and resolution would be lost as compared with the system which uses aluminum as a source.
In addition, it has been proposed by others that the CuL source could be used with a thin Al.sub.2 O.sub.3 or Si.sub.3 N.sub.4 mask. Unfortunately, these masks have a severe disadvantage of being extremely fragile, as well as requiring special polishing in preparation of the Al.sub.2 O.sub.3 mask to be suitable for use.
Also known in the art, are so-called "stencil" masks which are used in E-beam operations. However, the stencil masks have the distinct disadvantage that only "holes" can be produced--not "donuts". That is, a hollow type structure cannot be easily produced with a stencil mask operation. Also, production of high resolution stencil masks is difficult. Moreover, in E-beam operations, the Mylar membrane masks noted above relative to x-ray technologies are ineffective because of the attenuation of the E-beam.